Increased demand for sulphur-containing fertilizers stems from the discovery that low crop yields in certain cases may be related to deficiencies in sulphur in the soil. An example of a species with high sulphur requirements is Canola. Canola is an important cash crop in Alberta, Canada, and has high sulphur requirements at any growth stage. A shortage of sulphur can cause serious reductions in crop yield and quality.
Manufacturing processes for sulphur-containing fertilizers of the phosphate type often involve the use or incorporation of sulphates. A disadvantage of sulphates is that they are very mobile in the soil and easily leach out of the root zone, effectively making the sulphate nutrient unavailable to the plants.
Elemental sulphur is not leached out of the soil as readily as sulphates. Instead, micron-sized elemental sulphur particles (e.g. with size from 1 to 200 μm) are oxidized to sulphate sulphur, which is the form utilised by the plants, by soil bacteria during the cropping season. Elemental sulphur can therefore be considered a slow (timed) release form of plant nutrient sulphur that is less prone to leaching out of the crops root zone. It is, therefore, advantageous to have a large proportion of the sulphur in fertilizers present as elemental sulphur. Furthermore, elemental sulphur offers some additional benefits in agriculture, including acting as a fungicide against certain micro organisms, acting as a pesticide against certain soil and plant pests, assisting the decomposition of plant residues and improving phosphorus and nitrogen nutrient utilisation and reducing the pH of alkaline and calcareous soils.
Thus, it is advantageous to incorporate sulphur in sulphur-containing fertilizers as elemental sulphur.
Processes for the manufacture of sulphur-containing fertilizers, wherein elemental sulphur is used, are known in the art. NZ 213682 discloses a method for providing sulphur for use in a sulphur-containing fertilizer wherein liquid sulphur is added to phosphoric acid, wherein the phosphoric acid is in a high energy state of shear in a high energy vortex. This provides a dispersion of sulphur in phosphoric acid, which can be used in the formation of sulphur-containing triple superphosphate fertilizer. The sulphur dispersion is combined and reacted with phosphate rock, and the resulting material is granulated. U.S. Pat. No. 4,372,872 discloses a process wherein a suspension of sulphur is produced by agitating an aqueous medium with a high shear mixer, and introducing sulphur (in particulate or molten form) into the agitated medium. The sulphur suspensions can be applied to the soil. The present inventors have found that, with such methods of preparing sulphur for incorporation into fertilizer products, it is difficult to avoid providing relatively large sulphur particles and these larger particles lead to operational difficulties during fertilizer manufacture such as build-up of particles in the apparatus and possible plugging of the process equipment and process lines. Also, said relatively large sulphur particles are more difficult to incorporate into and bond to the other fertilizer material.
WO 2008/089568 discloses a method for wet grinding sulphur feedstock wherein hydrocyclones are used to separate elemental sulphur particles having selected size distributions. The product can be further processed to produce a sulphur-based fertilizer. In the examples of WO 2008/089568 a ball mill is used as a primary grinding stage and a Vertimill as a second grinding unit. A disadvantage of this wet grinding process is its process complexity (a number of pieces of process equipment is needed) and that the process is energy intensive. No mention of a dispersion mill is made.
Yalcin et al in Powder Technology 146 (2004), 193-199, describe a wet ball milling process for producing finely divided sulphur that can be used in a soil acidification process. Such milling processes require apparatus which may be complex and expensive as it must be corrosion resistant.
WO 2004/043878 discloses a process for the manufacture of sulphur-containing fertilizers comprising the steps of:
(a) mixing ammonia, phosphoric acid and water in a reactor unit to obtain an ammonium phosphate mixture;
(b) introducing the mixture obtained in step (a) into a granulator unit to obtain granules,
wherein a liquid phase comprising elemental sulphur is brought into contact with ammonia, phosphoric acid and water in the reaction unit in step (a) or is introduced in the granulator in step (b). No mention is made of milling the elemental sulphur.
U.S. Pat. No. 5,522,553 discloses a dispersion mill and more particularly a method and apparatus for producing liquid suspensions of finely divided matter, such as in the manufacture of paints, printing inks, lacquers, carbon paper coatings, in the treatment of waste water and the like. There is no suggestion in U.S. Pat. No. 5,522,553 to use the dispersion mill in a process for preparing an elemental sulphur-containing fertilizer.
The present inventors have sought to provide an alternative method for manufacturing elemental sulphur-containing fertilizer. In particular, the present inventors have sought to provide a simpler and less energy intensive process for manufacturing elemental sulphur-containing fertilizer.
In addition, the present inventors have sought to provide a process for manufacturing elemental sulphur-containing fertilizer, wherein the particle size of the elemental sulphur particles can be easily controlled.